The present invention relates to flat, high frequency cables having at least two longitudinal conductors spaced from each other and interconnected with each other along their entire lengths. The conductors are arranged in parallel to each other and held in position by insulating means. Such cables are used, for example, in transmitting high frequency signals from a transmitter to a receiver located at close proximity to each other in the so called near field. The transverse conductance in such cables is substantially lower than the conductivity of the longitudinal conductors.
Cables of this type are used, for example, for precisely measuring the distance between vehicles on a track, for instance, as disclosed in U.S. Pat. No. 3,836,770 granted on Sept. 17, 1974. In prior art cables of this type the transverse conductance is established by means of a layer of an electrically conducting synthetic material whereby especially synthetic foils are used. U.S. Pat. 3,809,803 granted May 7, 1974 describes a cable in which the transverse conductance is accomplished by a conducting foil made of a plastic material having a carbon coating thereon.
Such a cable satisfies the requirements of the use for which the cable is intended in an excellent manner. However, the manufacturing of such cables is expensive because substantial difficulties are encountered in providing suitable contact means between the longitudinal conductors and the electrically conducting carbon coated synthetic foil which must have a conductivity within narrow tolerance limits. Providing the necessary contacts between the longitudinal conductors and the carbon coated foil becomes especially involved and hence expensive if it is necessary to assure a uniform and reproducible constancy as well as a geometrical and time constancy. In other words, the contacts must remain where they are intended over prolonged periods of time. These latter conditions are especially hard to maintain where the cable is intended for various kinds of uses and not only for the distance measuring.